The Healthcare Commission has launched an investigation into apparently high mortality rates among emergency admissions at the Mid Staffordshire foundation trust.
But the trust says it has solved the problem, which was caused by poor clinical coding.
Commission head of investigations Nigel Ellis said: "An apparently high rate of mortality does not necessarily mean there are problems with safety. It may be there are other factors here, such as the way that information about patients is recorded by the trust. Either way, it does require us to ask questions, which is why we are carrying out this investigation."
The investigation will also look into patients' complaints about the quality of care given to older people in particular and the governance arrangements to protect patient safety.
There was no cause for immediate alarm, he added.
This week's decision to investigate follows an initial analysis of the data and two site visits.
The trust, which gained foundation status in February, said it was co-operating fully with the investigation.
Chief executive Martin Yeates said the trust's own investigation had highlighted problems with coding. He said: "Over the last year, we have employed more clinical coding experts to work within the various specialty departments to help staff to record information and improve the quality of data.
"As a result, in recent months, we believe there has been a significant improvement in the way our mortality rates are recorded."